This invention relates to improvements in the conversion of heat to electric energy, more particularly to the use of stacked layers of thin pyroelectric films with means for alternatingly delivering hot and cold fluids to the films. A novel voltage controller slave to a magnetic proximity switch mounted on a rotating flow controller, provides a synchronization of the thermal and electrical cycling of the pyroelectric converter.
The use of capacitors with temperature dependent dielectrics in pyroelectric systems is taught in U.S. Pat. No. 4,220,906, issued Sep. 2, 1980 to Drummond and U.S. Pat. No. 4,441,067, issued Apr. 3, 1984 to O""Hara. Both patents identify the suitability of such systems for operation with waste heat from industries such as at pulp and paper mills, steel works, petrochemical plants, glass manufacturers, and electric power stations.
Unfortunately when thermal efficiency is increased by extracting more heat from the waste heat, the cost of installing additional equipment for extra heat utilization often becomes prohibitive. Also, technical difficulties arise when extracting a large amount of heat from a process; when a system for highly efficient heat recovery from waste gases is planned, the condensation of acidic liquids and subsequent equipment corrosion become a serious technical hurdle to implementing waste heat utilization schemes. Further, when heat source temperature is low the available heat for further useful work becomes limited, being limited by Carnot cycle efficiency.
A pyroelectric film can function as a temperature-dependent capacitor; when heat is applied and its temperature increases, its capacitance (ability to store charge) diminishes. When the film temperature is raised charge can no longer remain on the surfaces of the film and is forced to leave, giving out electrical energy. Thus heat input is converted to electrical charge.
The present invention uses a pyroelectric conversion cycle discussed in Olsen, R. B., Brisco, J. M. Bruno, D. A. and Butler, W. F., xe2x80x9cA pyroelectric energy converter which employs regenerationxe2x80x9d, Ferroelectrics, vol. 38, pp. 975-978 (1981) and in Olsen, R. B., Bruno, D. A. and Brisco, J. M., xe2x80x9cPyroelectric conversion cycle of vinylidene fluoride-trifloroethylene copolymerxe2x80x9d, J. Appl. Phys. 57(11), pp 5036-5042 (1985).
U.S. Pat. No. 4,425,540, issued Jan. 10, 1987, and U.S. Pat. No. 4,647,836, issued Mar. 3, 1987, both to Olsen, disclose the same power cycle that can be used to convert waste heat to electricity directly, hereinafter referred to as the Olsen cycle. The thermal response of pyroelectric films is synchronized with externally controlled bias voltage in order to convert heat energy to electrical energy. This Olsen cycle parallels a heat engine.
Ferroelectric materials such as PZST (sintered ceramic of lead zirconate, lead titanate and lead stannate) and P(VDF-TrFE) (copolymers of vinylidene fluoride-trifluoroethylene) are suitable for pyroelectric conversion (U.S. Pat. No. 4,620,262, October 1986). Many monomers contain polar groups. To yield useful piezoelectric and also pyroelectric polymers, their constituents should not be so bulky as to prevent crystallization of the macromolecules or to force them into shapes (such as helical) that result in extensive internal compensation of polarization. The fluorine atom is very small, its van der Waals radius (1.35 xc3x85) being only slightly larger than that of hydrogen (1.2 xc3x85), and it forms highly polar bonds with carbon, having dipole moment 6.4xc3x9710xe2x88x9230 Coulomb-meter (=1.92 debye). Common resulting polyfluorocarbons are polyvinylidene fluoride (PVF2), poly vinyl fluoride (PVF) and polytrifluoroethylene (PF3D). Other polar groups that could produce useful piezoelectric and pyroelectric polymers include Cxe2x80x94Cl bond with 2.1 debye, Cxe2x80x94CN bond with 3.86 debye, and Cxe2x95x90Oxe2x80x94Hxe2x80x94N containing a highly polar hydrogen bond at 3.59 debye. When PVF2 is cooled it forms a number of crystallized phases. However, without a poling process the fluorine atoms take largely at trans and gauche positions, thus overall polarity remains neutral. The most useful phase is known as xcex2-phase in PVF2 can be increased by applying external electric field on a stretched PVF2 film.
Copolymers of vinylidene fluoride and trifluorethylene P(VF2-TrFE) have particularly useful property. They commonly contain 20 to 30 mol TrFE. When they are cooled from melting temperatures to room temperature, they form a xcex2-like phase without stretching the polymers. This is because trifloroethylene (CF2xe2x80x94CHF)n contains a greater proportion of the comparatively bulky fluorine atoms than PVF2, their molecular chains cannot accommodate the tg+tgxe2x88x92 conformation and are therefore forced to crystallize directly with the more extended all-trans conformation. Further, the TrFE in the P(VF2-TrFE) chain appears to stabilize the xe2x80x9ctransxe2x80x9d form to the degree just suitable for the conformational change when exposed to reverse external field.
P(TrFE-VF2) copolymers from 12.5 to 85 mol % VF2 always show the xcex2-phase crystal (trans or trans-like conformation), and do not transform into the non-polar xcex1-phase by any thermal treatments. Particularly those between 65 and 80 mol % VF2 spontaneously crystallize into all-trans xcex2-structure (ferroelectric) with a high degree of crystallinity without the need for drawing. They undergo a ferroelectric phase transition at a Curie temperature from 60xc2x0 C. to 140xc2x0 C. with increasing VF2 content.
The major process parameters determining the power output from a pyroelectric film are the volumetric resistivity of the pyroelectric material, the temperature dependency of the pyroelectric coefficient of a given film, the span of temperature cycling (the span of high and low film temperatures), the width of operating voltages (the difference between Vhigh and Vlow) and the frequency of the Olsen cycle.
U.S. Pat. No. 4,647,836 shows that the overall system efficiency of pyroelectric conversion increases with the use of a heat regeneration technique, see also Olsen, R. B. and Brown, D. D., xe2x80x9cHigh efficiency direct conversion of heat to electrical energy-related pyroelectric measurementsxe2x80x9d, Ferroelectrics, vol. 40, pp. 17-27 (1982). One of the important parameters that influences overall system efficiency is the regeneration of usable heat. It is critically important to convert as much heat as possible to electricity before heat degrades to an unusable state. Heat regeneration allows this. When pyroelectric films receive heat from heating fluid at a higher temperature, their temperature rises. Thus they themselves become a heat source with respect to fluid at a lower temperature. When this sequence is repeated a given amount of heat can be shuttled many times via the pyroelectric film assemblies between heat source and heat sink temperatures before the heat eventually degrades and the heat shuttling becomes impossible. Previous designs were bulky as well as unsuitable for rapid thermal cycling due to the reciprocal motion of heat transfer fluids in heat exchangers housing pyroelectric films.
The present invention provides an improved apparatus and method for converting heat to electrical energy with a multi-layered stack of thin pyroelectric films which overcome problems with prior bulky designs. The pyroelectric film surface is cycled between higher and lower temperatures by the displacement of higher and lower temperature fluid over the film surface. The design according to the invention alternates hot and cold fluid without reversing direction of the fluids over the films. Rapid thermal cycling is achieved with this uni-directional fluid flow. A novel voltage controller slave to a magnetic proximity switch mounted on a rotating flow controller, provides a synchronization of the thermal and electric cycling of the pyroelectric films.